Photo-chromic lenses are lenses that darken when exposed to sunlight to reduce the brightness of the ambient sunlight reaching the eyes. Typically, the darkening effect of photo-chromic lenses is triggered by the UV radiation present in sunlight, and the darkening effect attenuates light. Once the UV radiation is removed, the photo-chromic molecules, which increase their absorption during exposure to UV light, transition back to a substantially transparent or non-darkened state. Artificial indoor light does not ordinarily include UV radiation. As such, photo-chromic lenses will reversibly darken when outdoors, and return to their non-darkened state when indoors.
Prescription eyeglasses are available with photo-chromic lens darkening. For glass lenses, the photo-chromic molecules (e.g., silver halides such as silver chloride) are often embedded throughout the volume of the lens substrate (not coated), while plastic lenses typically use a layer of organic photo-chromic molecule (e.g., oxazines) that is coated on a single surface of the plastic lens to achieve reversible darkening. Conventionally, the photo-chromic coating is spin coated on the single forward facing side surface (ambient scene side) of the plastic lens.